1. Field of the Invention
Embodiments of the present invention relate to an image forming device, such as a copier, a printer, a facsimile machine, and a combined machine thereof.
2. Description of the Related Art
An electrophotographic image forming device, such as a copier, a printer, a facsimile machine, or a combined machine thereof includes a fixing device for fusing and fixing toner (developer) adhered to a sheet of paper. For the fixing device, a temperature required for fixing a toner image (a fixing temperature), which is not fixed onto the sheet of paper, has been set in advance. However, the required temperature varies depending on a type of the toner image or a type of the sheet of paper.
As a factor that causes the fixing temperature to vary, for example, density of toner adhered to the sheet of paper, and an amount of isolated toner dots adhered to the sheet of paper can be considered. When a coverage rate is high and the amount of the isolated toner dots is large, a higher fixing temperature may be required, compared to a case in which the coverage rate is low and the amount of the isolated toner dots is small. Therefore, for a conventional image forming device, a target fixing temperature is set based on the worst condition for fixing the toner image.
However, if the condition for fixing, which is equivalent to the fixing condition for the case in which it is difficult to fix the toner image, is applied in a case in which it is easy to fix a toner image, the fixing temperature is maintained at an unnecessarily high temperature. Thus, the power is unnecessarily consumed in a heating device, contradicting the requirement of reducing the energy consumption in recent years.
Therefore, techniques have been proposed as described below. Namely, the techniques are for suitably switching the condition for fixing, between a case in which a toner image is easily fixed and a case in which a toner image is not easily fixed. For example, Patent Document 1 (Japanese Published Unexamined Application No. 2009-53421) discloses a technique for optimizing an amount of heat for fixing a toner image by adjusting a nip width depending on an amount of toner used for an image to be fixed.
Further, Patent Document 2 (Japanese Published Unexamined Application No. 2006-133580) discloses a technique for raising the fixing temperature for a low toner consumption mode (a mode for reducing toner consumption) for performing area coverage modulation processing, in which a unit that analyzes data on a pixel-by-pixel basis and applies dithering to, for example, a black image that includes many pixels and a unit that generates a halftone image by controlling a time period for emitting laser light on a dot-by-dot basis are utilized.
Further, Patent Document 3 (Japanese Registered Patent No. 3295273) discloses a technique for switching a start-up time for fixing between a text mode and a photo mode. Further, Patent Document 4 (Japanese Published Unexamined Application No. 2008-185638) discloses a technique for controlling the fixing temperature by determining whether colored fine pixels are isolatedly arranged. Further, Patent Document 5 (Japanese Published Unexamined Application No. 2008-268784) discloses a technique that raises an optimum fixing temperature for a portion in which a coverage rate is high, such as a photograph, compared to that of a portion in which a coverage rate is low, such as text. Additionally, Patent Document 5 discloses that coverage rates are calculated for a photo area and a text area, respectively, and that the fixing temperatures are individually optimized for the photo area and the text area. Further, Patent Document 6 (Japanese Published Unexamined Application No. 2009-181065) discloses a technique for controlling the fixing temperature depending on image density information based on a data dot number per a predetermined area, so as to optimize the fixing temperature depending on image information.
The above patent documents disclose configurations that can optimize the temperature, for a case in which one sheet of recording medium is fed through a device, or for a case in which sheets of recording media are fed through a device in a constant fixing mode (e.g., the photo mode, or the text mode). However, for these configurations, a case is not considered, in which the fixing temperature is varied on a sheet-by-sheet basis while sheets of recording media are continuously fed. Thus, these configurations are not suitable for performing temperature control on a sheet-by-sheet basis during continuous feeding of the sheets of recording media.
Specifically, in Patent Document 1, it is described that the amount of heat for fixing is optimized by adjusting the nip width depending on the amount of the toner included in the image to be fixed. However, it is quite difficult to adjust the nip width for fixing on a sheet-by-sheet basis during continuous feeding of the sheets of recording media. Since the processing speeds of recent image forming devices are increasing, it is not realistic to perform this control on a sheet-by-sheet basis during continuous feeding.
Further, the techniques disclosed in Patent Documents 2 and 3 are for controlling the fixing temperature depending on the selected mode. Thus, for these configurations, it is not considered to control the fixing temperature on a sheet-by-sheet basis during continuous feeding of the sheets of recording media.
Patent Document 4 discloses the technique for controlling the fixing temperature by determining whether the colored fine pixels are isolatedly arranged. In the technique, when an image is divided into pixels having small areas, and the pixels having small areas are further divided into fine pixels, the fixing control is varied depending on colored areas of the fine pixels. Hence, when the temperature control is performed on a sheet-by-sheet basis during continuous feeding, an amount of information per one sheet becomes huge. Therefore, it becomes difficult to determine to what extent the fixing temperature can be decreased. Even if the control is possible, the load on the information processing is huge.
Further, Patent Document 5 merely discloses the technique for controlling the fixing temperature depending on the area of the image, when an output image, in which photo images and text images are mixed, is fixed. Patent Document 5 does not propose a technique for performing temperature control on a sheet-by-sheet basis during continuous feeding.
Further, Patent Document 6 discloses the technique for controlling the fixing temperature depending on the image density information, which is based on the data dot number per the predetermined area, so as to optimize the fixing temperature depending on the image data. However, in this case, similar to the case of Patent Document 4, an amount of information per one sheet becomes huge, and it is difficult to determine to what extent the fixing temperature can be decreased. Even if the control is possible, the load on the information processing is huge.